Tahun 2000. Part IV processing by the removal of heat

21.2.2 Cooled-liquid freezers

In immersion freezers, packaged food is passed through a bath of refrigerated propylene glycol, brine, glycerol or calcium chloride solution on a submerged mesh conveyor. In contrast with cryogenic freezing (Section 21.2.4), the liquid remains fluid throughout the freezing operation and a change of state does not occur. The method has high rates of heat transfer (Table 21.3) and capital costs are relatively low. It is used commercially for concentrated orange juice in laminated card–polyethylene cans, and to pre-freeze film- wrapped poultry before blast freezing.

Fig. 21.3 Spiral freezer, self-stacking belt.

(Courtesy of Frigoscandia Ltd.)

21.2.3 Cooled-surface freezers

Plate freezers consist of a vertical or horizontal stack of hollow plates, through which refrigerant is pumped at 40ºC (Fig. 21.4). They may be batch, semi-continuous or continuous in operation. Flat, relatively thin foods (for example filleted fish, fish fingers or beefburgers) are placed in single layers between the plates and a slight pressure is applied by moving the plates together. This improves the contact between surfaces of the food and the plates and thereby increases the rate of heat transfer. If packaged food is frozen in this way, the pressure prevents the larger surfaces of the packs from bulging. Production rates range from 90–2700 kg h ¹ in batch freezers. Advantages of this type of equipment include good economy and space utilisation, relatively low operating costs compared with other methods, little dehydration of the product and therefore minimum defrosting of condensers, and high rates of heat transfer (Table 21.3). The main disadvantages are the relatively high capital costs, and restrictions on the shape of foods to those that are flat and relatively thin.

Scraped-surface freezers are used for liquid or semi-solid foods (for example ice cream). They are similar in design to equipment used for evaporation (Chapter 13, Fig.

13.5) and heat sterilisation (Chapter 12) but are refrigerated with ammonia, brine, or other refrigerants. In ice cream manufacture, the rotor scrapes frozen food from the wall of the freezer barrel and simultaneously incorporates air. Alternatively, air can be injected into the product. The increase in volume of the product due to the air is expressed as overrun (see Chapter 1, Section 1.1.1).

Freezing is very fast and up to 50% of the water is frozen within a few seconds (Jaspersen, 1989). This results in very small ice crystals, which are not detectable in the mouth and thus gives a smooth creamy consistency to the product. The temperature is reduced to between 4ºC and 7ºC and the frozen aerated mixture is then pumped into



Fig. 21.4 Plate freezer.

(Courtesy of Frigoscandia Ltd. and Garthwaite, A. (1995).)

containers and freezing is completed in a ‘hardening room’ (see ‘chest freezers’ above). Further details of ice cream production are given in Chapter 4.

21.2.4 Cryogenic freezers

Freezers of this type are characterised by a change of state in the refrigerant (or cryogen) as heat is absorbed from the freezing food. The heat from the food therefore provides the latent heat of vaporisation or sublimation of the cryogen. The cryogen is in intimate contact with the food and rapidly removes heat from all surfaces of the food to produce high heat transfer coefficients and rapid freezing. The two most common refrigerants are liquid nitrogen and solid or liquid carbon dioxide. Dichlorodifluoromethane (refrigerant

12 or Freon 12) was also previously used for sticky or fragile foods that stuck together in

clumps (for example meat paste, shrimps, tomato slices), but its use has now been phased out under the Montreal Protocol, due to its effects on the earth’s ozone layer (further details are given in Chapter 19).

The choice of refrigerant is determined by its technical performance for a particular product, its cost and availability, environmental impact and safety (Heap, 1997). The market for frozen foods is increasingly characterised by shorter product life cycles and hence more rapid changes to the number and type of new products. There is a significant commercial risk if the payback period on capital investment exceeds the product life cycle, unless the equipment is sufficiently flexible to accommodate new products (Summers, 1998). Two advantages of cryogenic freezers, compared to mechanical systems, are the lower capital cost and flexibility to process a number of different products without major changes to the system (Miller, 1998).

Both liquid-nitrogen and carbon dioxide refrigerants are colourless, odourless and inert. When liquid nitrogen is sprayed onto food, 48% of the total freezing capacity (enthalpy) is taken up by the latent heat of vaporisation needed to form the gas (Table

21.4). The remaining 52% of the enthalpy is available in the cold gas, and gas is therefore

recirculated to achieve optimum use of the freezing capacity. Carbon dioxide has a lower enthalpy than liquid nitrogen (Table 21.4) but most of the freezing capacity (85%) is available from the subliming solid and the lower boiling point produces a less severe thermal shock. In addition, solid carbon dioxide in the form of a fine snow sublimes on contact with the food, and gas is not recirculated. Carbon dioxide is a bacteriostat but is also toxic, and gas should be vented from the factory to avoid injury to operators. Carbon dioxide consumption is higher than liquid-nitrogen consumption, but storage losses are lower.

Table 21.4 Properties of food cryogens
Property Liquid nitrogen Carbon dioxide

_{Density (kg m} ³<sub>) 784 464

Specific heat (kJ kg</sub> ¹ _K ¹<sub>) 1.04 2.26

Latent heat (kJ kg</sub> ¹_{) 358 352}

Total usable refrigeration effect (kJ kg ¹) 690 565

Boiling point (ºC) 196 78.5 (sublimation)
Thermal conductivity (W m ¹ K ¹) 0.29 0.19

Consumption per 100 kg of product frozen (kg) 100–300 120–375
From Graham (1984).
In liquid-nitrogen freezers, packaged or unpackaged food travels on a perforated belt through a tunnel (Fig. 21.5), where it is frozen by liquid-nitrogen sprays and by gaseous nitrogen. Production rates are 45–1550 kg h ¹. The temperature is either allowed to equilibrate at the required storage temperature (between 18ºC and 30ºC) before the food is removed from the freezer, or alternatively food is passed to a mechanical freezer to complete the freezing process. The use of gaseous nitrogen reduces the thermal shock to the food, and recirculation fans increase the rates of heat transfer. A newer design of tunnel, with fans located beneath the conveyor to produce gas vortices is described by Summers (1998). This design is said to double the output of conventional freezers of the same length, reduce nitrogen consumption by 20% and reduce already low levels of dehydration by 60%. The temperature and belt speed are controlled by microprocessors to maintain the product at a pre-set exit temperature, regardless of the heat load of incoming food. The equipment therefore has the same efficiency at or below its rated capacity. This results in greater flexibility and economy than mechanical systems, which have a fixed rate of heat extraction (Tomlins, 1995).

Other advantages include:

 simple continuous operation with relatively low capital costs (approximately 30% of the capital cost of mechanical systems)

 smaller weight losses from dehydration of the product (0.5% compared with 1.0–8.0%

in mechanical air-blast systems)

 rapid freezing (Table 21.3) which results in smaller changes to the sensory and nutritional characteristics of the product

 the exclusion of oxygen during freezing

 rapid startup and no defrost time

 low power consumption (Leeson, 1987).
The main disadvantage is the relatively high cost of refrigerant (nitrogen and carbon dioxide consumption are shown in Table 21.4).

Liquid nitrogen is also used in spiral freezers (Section 21.2.1) instead of vapour recompression refrigerators. The advantages include higher rates of freezing, and smaller

Fig. 21.5 Liquid-nitrogen freezer.
units for the same production rates because heat exchanger coils are not used. Other applications include rigidification of meat for high-speed slicing (Chapter 4), surface hardening of ice cream prior to chocolate coating (Chapter 23) and crust formation on fragile products such as seafood and sliced mushrooms (Londahl and Karlsson (1991), before finishing freezing in mechanical or cryogenic freezers. Other applications are described by Tomlins (1995).

Immersion of foods in liquid nitrogen produces no loss in product weight but causes a high thermal shock. This is acceptable in some products (for example raspberries, shrimps and diced meat), but in many foods the internal stresses created by the extremely high rate of freezing cause the food to crack or split. The rapid freezing permits high production rates of IQF foods using small equipment (for example a 1.5 m long bath of liquid nitrogen freezes 1 t of small-particulate food per hour).

21.3 Changes in foods
21.3.1 Effect of freezing

The main effect of freezing on food quality is damage caused to cells by ice crystal growth. Freezing causes negligible changes to pigments, flavours or nutritionally important components, although these may be lost in preparation procedures (Chapters 3 and 10) or deteriorate later during frozen storage. Food emulsions (Chapter 4) can be destabilised by freezing, and proteins are sometimes precipitated from solution, which prevents the widespread use of frozen milk. In baked goods a high proportion of amylopectin is needed in the starch to prevent retrogradation and staling during slow freezing and frozen storage.

There are important differences in resistance to freezing damage between animal and plant tissues. Meats have a more flexible fibrous structure which separates during freezing instead of breaking, and the texture is not seriously damaged. In fruits and vegetables, the more rigid cell structure may be damaged by ice crystals. The extent of damage depends on the size of the crystals and hence on the rate of heat transfer (Section

21.1.1). However, differences in the variety and quality of raw materials and the degree of control over pre-freezing treatments both have a substantially greater effect on food quality than changes caused by correctly operated freezing, frozen storage and thawing procedures. Details of the changes to meats are described by Devine et al. (1996) and changes to vegetables are described by Cano (1996).

The influence of freezing rate on plant tissues is shown in Fig. 21.6. During slow

freezing, ice crystals grow in intercellular spaces and deform and rupture adjacent cell walls. Ice crystals have a lower water vapour pressure than regions within the cells, and water therefore moves from the cells to the growing crystals. Cells become dehydrated and permanently damaged by the increased solute concentration and a collapsed and deformed cell structure. On thawing, cells do not regain their original shape and turgidity. The food is softened and cellular material leaks out from ruptured cells (termed ‘drip loss’). In fast freezing, smaller ice crystals form within both cells and intercellular spaces. There is little physical damage to cells, and water vapour pressure gradients are not formed; hence there is minimal dehydration of the cells. The texture of the food is thus retained to a greater extent (Fig. 21.6(b)). However, very high freezing rates may cause stresses within some foods that result in splitting or cracking of the tissues. These changes are discussed in detail by Spiess (1980).

Fig. 21.6 Effect of freezing on plant tissues: (a) slow freezing; (b) fast freezing.

(After Meryman (1963).)

21.3.2 Effects of frozen storage

In general, the lower the temperature of frozen storage, the lower is the rate of micro- biological and biochemical changes. However, freezing and frozen storage do not inactivate enzymes and have a variable effect on micro-organisms. Relatively high storage tem- peratures (between 4ºC and 10ºC) have a greater lethal effect on micro-organisms than do lower temperatures (between 15ºC and 30ºC). Different types of micro-organism also vary in their resistance to low temperatures; vegetative cells of yeasts, moulds and gram- negative bacteria (for example coliforms and Salmonella species) are most easily destroyed; Gram-positive bacteria (for example Staphylococcus aureus and Enterococci) and mould spores are more resistant, and bacterial spores (especially Bacillus species and Clostridium species such as Clostridium botulinum) are virtually unaffected by low temperatures. The majority of vegetables are therefore blanched to inactivate enzymes and to reduce the

Fig. 21.7 Effect of storage temperature on sensory characteristics.

(After Jul (1984).)

numbers of contaminating micro-organisms (Chapter 10). In fruits, enzyme activity is controlled by the exclusion of oxygen, acidification or treatment with sulphur dioxide.

At normal frozen storage temperatures ( 18ºC), there is a slow loss of quality owing to both chemical changes and, in some foods, enzymic activity. These changes are accelerated by the high concentration of solutes surrounding the ice crystals, the reduction in water activity (to 0.82 at 20ºC in aqueous foods) and by changes in pH and redox potential. The effects of storage temperature on food quality are shown in Fig. 21.7. If enzymes are not inactivated, the disruption of cell membranes by ice crystals allows them to react to a greater extent with concentrated solutes.

The main changes to frozen foods during storage are as follows:
 Degradation of pigments. Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degraded to brown pheophytin even in blanched vegetables. In fruits, changes in pH due to precipitation of salts in concentrated solutions change the colour of anthocyanins.

 Loss of vitamins. Water-soluble vitamins (for example vitamin C and pantothenic

acid) are lost at sub-freezing temperatures (Table 21.5). Vitamin C losses are highly
Table 21.5 Vitamin losses during frozen storage
Product Loss (%) at 18ºC during storage for 12 months

	Vitamin C	Vitamin ^B1	Vitamin ^B2	Niacin	Vitamin ^B6	Pantothenic acid	Carotene
Beans (green)	52	0–32	0	0	0–21	53	0–23
Peas	11	0–16	0–8	0–8	7	29	0–4
Beef steaks^a		8	9	0	24	22	–
Pork chops^a Fruit^b		18	0–37	5	0–8	18	–
Mean	18	29	17	16	–	–	37
Range	0–50	0–66	0–67	0–33	–	–	0–78

_{, apparent increase.}

^a _{Storage for 6 months.}

^b Mean results from apples, apricots, blueberries, cherries, orange juice concentrate (rediluted), peaches, raspberries and strawberries; storage time not given.

Adapted from Burger (1982) and Fennema (1975b).

temperature dependent; a 10ºC increase in temperature causes a sixfold to twentyfold increase in the rate of vitamin C degradation in vegetables and a thirtyfold to seventyfold increase in fruits (Fennema, 1975b). Losses of other vitamins are mainly due to drip losses, particularly in meat and fish (if the drip loss is not consumed).

 Residual enzyme activity. In vegetables which are inadequately blanched or in fruits,

the most important loss of quality is due to polyphenoloxidase activity which causes browning, and lipoxygenases activity which produces off-flavours and off-odours from lipids and causes degradation of carotene. Proteolytic and lipolytic activity in meats may alter the texture and flavour over long storage periods.

 Oxidation of lipids. This reaction takes place slowly at 18ºC and causes off-odours

and off-flavours.
These changes are discussed in detail by Fennema (1975a, 1982, 1996) and Rahman

(1999).
Recrystallisation

Physical changes to ice crystals (for example changes in their shape, size or orientation) are collectively known as recrystallisation and are an important cause of quality loss in some foods. There are three types of recrystallisation in foods as follows:
1. Isomass recrystallisation. This is a change in surface shape or internal structure, usually resulting in a lower surface-area-to-volume ratio.

2. Accretive recrystallisation. Two adjacent ice crystals join together to form a larger crystal and cause an overall reduction in the number of crystals in the food.

3. Migratory recrystallisation. This is an increase in the average size and a reduction in

the average number of crystals, caused by the growth of larger crystals at the expense of smaller crystals.

Migratory recrystallisation is the most important in most foods and is largely caused by fluctuations in the storage temperature. When heat is allowed to enter a freezer (for example, by opening a door and allowing warm air to enter), the surface of the food nearest to the source of heat warms slightly. This causes ice crystals to melt partially; the larger crystals become smaller and the smallest (less than 2 m) disappear. The melting crystals increase the water vapour pressure, and moisture then moves to regions of lower
vapour pressure. This causes areas of the food nearest to the source of heat to become dehydrated. When the temperature falls again, water vapour does not form new nuclei but joins onto existing ice crystals, thereby increasing their size. There is therefore a gradual reduction in the numbers of small crystals and an increase in the size of larger crystals, resulting in loss of quality similar to that observed in slow freezing.

Cold stores have a low humidity because moisture is removed from the air by the refrigeration coils (see psychrometrics in Chapter 15). Moisture leaves the surface of the food to the storage atmosphere and produces areas of visible damage known as freezer burn. Such areas have a lighter colour due to microscopic cavities, previously occupied by ice crystals, which alter the wavelength of reflected light. Freezer burn is a particular problem in foods that have a large surface-area-to-volume ratio (for example IQF foods) but is minimised by packaging in moisture-proof materials (Chapter 24). The causes of dehydration during freezing and frozen storage are discussed in detail by Norwig and Thompson (1984).

Temperature fluctuations are minimised by:
 accurate control of storage temperature (±1.5ºC)

 automatic doors and airtight curtains for loading refrigerated trucks

 rapid movement of foods between stores

 correct stock rotation and control.

These techniques, and technical improvements in handling, storage and display equip- ment, have substantially improved the quality of frozen foods (Jul, 1984).
Storage life

There is some confusion and lack of precise information on the storage life of frozen foods, caused in part by the use of different definitions. For example a European Community directive states that frozen storage must ‘preserve the intrinsic characteristics’ of foods, whereas the International Institute of Refrigeration defines storage life as ‘the physical and biochemical reactions . . . leading to a gradual, cumulative and irreversible reduction in product quality, such that after a period of time the product is no longer suitable for consumption . . . ’. Another definition by Bogh-Sorensen describes practical storage life (PSL) as ‘the time the product can be stored and still be acceptable to the consumer’ (Evans and James, 1993). These definitions differ in the extent to which a product is said to be acceptable and rely heavily on the ability of taste panellists to detect changes in flavour, aroma, etc. that can be used to measure acceptability.

The use of PSL and to a lesser extent, the concept of high-quality life (HQL), is used to establish storage life. PSL is defined as ‘the time that a statistically significant difference (P 0.01) in quality can be established by taste panellists’. These methods therefore measure the period that food remains essentially the same as when it was frozen. This should not be confused with a storage life that is acceptable to consumers as foods may be acceptable for three to six times longer than the PSL or HQL. Examples of PSL for meats and HQL for vegetables, stored at three temperatures are shown in Table 21.6.

The main causes of loss of storage life are fluctuating temperatures and the type of

packaging used. Other factors, including type of raw material, pre-freezing treatments and processing conditions are discussed in detail by Evans and James (1993). Temperature fluctuation has a cumulative effect on food quality and the proportion of PSL or HQL lost can be found by integrating losses over time. Time-temperature tolerance (TTT) and product-processing-packaging (PPP) concepts are used to monitor
Table 21.6 Storage life of meats measured by PSL and vegetables measured by HQL
Product Practical storage life (PSL) (months)

12ºC 18ºC 24ºC

Beef carcasses	8	15	24
Ground beef	6	10	15
Veal carcasses	6	12	15
Lamb carcasses	18	24	24
Pork carcasses	6	10	15
Sliced bacon	12	12	12
Chicken, whole	9	18	24
Turkey, whole	8	15	24
Ducks, geese, whole	6	12	18
Liver	4	12	18

High quality life (HQL) (months)
7ºC 12ºC 18ºC

Green beans	1	3.1	9.8
Cauliflower	0.4	2	9.7
Peas	1	3	10.1
Spinach	0.76	1.9	6.2

_{From Guadagni (1968) and Evans and James (1993).}

and control the effects of temperature fluctuations on frozen food quality during production, distribution and storage (Olsson, 1984; Bogh-Sorensen, 1984).

Coloured indicators are being developed to:
 show the temperature of food (for example, liquid crystal coatings which change colour with storage temperature)

 indicate temperature abuse (for example wax melts and releases a coloured dye when

an unacceptable increase in temperature occurs)

 integrate the time–temperature combination that a food has received after packaging and to give an indication of the remaining shelf life (Fig. 21.8).

In the last category, indicators may contain a material that polymerises as a function of time and temperature to produce a progressive, predictable and irreversible colour change. In another type, a printed label contains diacetylene in the centre of a ‘bull’s eye’, with the outer ring printed with a stable reference colour. The diacetylene gradually darkens in colour due to combined time and temperature and when it matches the reference ring the product has no remaining shelf life. An example of a time–temperature integrator, based on an enzymic reaction which changes the colour of a pH indicator, is described by Blixt (1984) and Selman (1995) has reviewed developments in this area. More recently a bar code system has been developed that is applied to a pack as the product is dispatched. The bar code contains three sections: a code giving information on the product identity, date of manufacture, batch number, etc. to identify each container uniquely. A second code identifies the reactivity of a time–temperature indicator and the third section contains the indicator material. When the bar code is scanned by a hand-held microcomputer, a display indicates the status and quality of the product with a variety of pre-programmed messages (for example: ‘Good’, ‘Don’t use’ or ‘Call QC’). A number of microcomputers can be linked via modems to a central control computer, to produce a portable monitoring system that can track individual containers throughout a distribution chain.